Most purposebuilt security systems are limited to only a few specific device models, because that’s how they get “rocksolid stability.” As soon as you open the system up to custom code, adding devices that aren’t officially tested, and cloud to cloud integrations, you introduce multiple points of potential failure.
So the demands of a security system are usually very different then the demands of a home automation system. With home automation, users always want to add the latest and greatest new device. So there’s a decision you need to make very early on, which is whether you want to go for security system levels of reliability or not. If so, I would look at the purposebuilt systems. There are a number of inexpensive ones with no monitoring or optional monitoring that are very stable.
You also need to make the decision early on about whether you want cameras or not, because some of the least expensive systems don’t offer video monitoring, they just use contact and motion sensors.
One of the most interesting newer systems is abode. The cameras aren’t great quality, but they work OK. The amount of home automation is very limited, even the number of rules you can have, but that’s how they are keeping the security side stable.
https://www.amazon.com/abode-Connected-Security-Automation-Starter/dp/B016S34EQ8
@SBDOBRESCU uses both abode and SmartThings, he would probably have more to add.
Simplisafe used to be a popular choice and quite a few community members have it, but they have recently changed their business model and they now charge for a lot of things that used to be free so I’m not sure it would meet your requirements. You can find community members who have used both it and SmartThings and so can say more about comparative stability in the following thread: